Solid oxide fuel cells hold promise for a variety of power applications including distributed power generation and vehicular use. Present SOFC systems are capable of operating at substantially higher temperatures than polymer electrolyte or direct alcohol fuel cell systems, being able to withstand temperatures of as high at 1000° C. Moreover, SOFC are substantially more tolerant of “contaminant” gases that often accompany the hydrogen fuel, particularly when produced from a hydrocarbon source. The present invention integrates temperature swing reforming with a solid oxide fuel cell to provide an efficient power generation system that can be fueled with common hydrocarbon fuel.
Conventional synthesis gas generating processes include steam reforming, gas phase partial oxidation and autothermal reforming. Each of these processes has advantages and disadvantages when compared to each other.
In a steam reforming process, steam is reacted with a hydrocarbon containing feed to produce a hydrogen-rich synthesis gas. The general stoichiometry, as illustrated for methane, is:CH4+H2O→CO+3H2  (1)Typically, an excess of steam is used to drive the equilibrium to the right. As applied to hydrogen manufacture, excess steam also serves to increase the water-gas shift reaction:CO+H2O→CO2+H2  (2)
Because of the high endothermicity of the reaction, steam reforming is typically carried out in large furnaces, in which a reforming catalyst is packed into tubes. The tubes must withstand the high pressure of the produced synthesis gas, while transmitting heat at temperatures approaching 1000° C. As described in Stanford Research Institute International Report No. 212 (1994), steam reforming process efficiency, (defined as the heat of combustion of product hydrogen divided by the heat of combustion of reforming feed and furnace fuel), is approximately 74%, while the space velocity, (defined as Standard Cubic Feet per Hour of C1-equivalent feed/ft3 of catalyst bed) is about 1000 hr−1. Unfortunately, steam reforming furnaces occupy a very large volume of space, substantially greater than the tube volume. This feature, and the relatively low efficiency, combine to severely limit its utility in point-of-use fuel applications such as fuel cells and would likely be unfeasible for on-board vehicle applications or distributed power applications.
Sederquist (U.S. Pat. Nos. 4,200,682, 4,240,805, 4,293,315, 4,642,272 and 4,816,353) all teach a steam reforming process in which the heat of reforming is provided within the bed by cycling between combustion and reforming stages of a cycle. As noted by Sederquist, high quality heat recovery within reforming bed can produce results in a theoretical efficiency of about 97%. However, these patents describe a process that operates at very low productivity, with space velocities of around 100 hr−1. One consequence of Sederquist's low space velocity is that resulting high heat losses impede their ability to achieve high efficiency. The present invention solves this problem.
Oxygen ion conducting solid oxide fuel cells typically operate at temperatures ranging from about 650° C. to about 1000° C. These temperatures are required for the oxide ion to be sufficiently mobile in the electrolyte.
The inventors here have discovered a process for producing hydrogen from a hydrocarbon containing fuel integrated with a proton conducting solid oxide fuel cell that produces a highly efficient power generating system.